Gas phase detection and rotational spectroscopy of ethynethiol, HCCSH

We report the gas-phase detection and spectroscopic characterization of ethynethiol ($\mathrm{H}\mathrm{C}\mathrm{C}\mathrm{S}\mathrm{H}$), a metastable isomer of thioketene (${\mathrm{H}}_2{\mathrm{C}}_2\mathrm{S}$) using a combination of Fourier-transform microwave and submillimeter-wave spectroscopies. Several $a$-type transitions of the normal species were initially detected below 40 GHz using a supersonic expansion-electrical discharge source, and subsequent measurement of higher-frequency, $b$-type lines using double resonance provided accurate predictions in the submillimeter region. With these, searches using a millimeter-wave absorption spectrometer equipped with a radio frequency discharge source were conducted in the range 280 - 660 GHz, ultimately yielding nearly 100 transitions up to ${}^r{R}_0(36)$ and ${}^r{Q}_0(68)$. From the combined data set, all three rotational constants and centrifugal distortion terms up to the sextic order were determined to high accuracy, providing a reliable set of frequency predictions to the lower end of the THz band. Isotopic substitution has enabled both a determination of the molecular structure of $\mathrm{H}\mathrm{C}\mathrm{C}\mathrm{S}\mathrm{H}$ and, by inference, its formation pathway in our nozzle discharge source via the bimolecular radical-radical recombination reaction $\mathrm{S}\mathrm{H}+{\mathrm{C}}_2\mathrm{H}$, which is calculated to be highly exothermic (-477 kJ/mol) using the HEAT345(Q) thermochemical scheme.